THE PHYSIOLOGY OF THE HEARTBEAT 185 



ventricular node and bundle measure about 15 mm. in length, and about 

 2 mm. in width. 



The rest of the tissue between the auricles and ventricles is fibrous 

 in nature, although other connections like those of the auriculoventricular 

 bundle have been described by Kent. One of these, called the right lat- 

 eral connection, runs between the right auricle and the external wall of 

 the right ventricle. 



Another, but much smaller, mass of similar embryonic cardiac tissue 

 has more recently been discovered by Keith and Flack in the parts of 

 the auricle which correspond anatomically to the sinus venosus of the 

 heart of cold-blooded animals that is, in the area lying between the 

 openings of the vena cavae and around the coronary sinus. To be more 

 explicit, this tissue lies "in the sulcus terminalis just below the fork 

 formed by the junction of the upper surface of the auricular appendix 

 with, the superior vena cava." This sinoauricular node, as it is called, 

 is more or less club-shaped, the blunt end of the club being above, as 

 shown in the accompanying figure (Figs. 51 and 52). It is important to 

 note that there is no direct connection visible between the sinoauricular 

 and auriculoventricular nodes (Fig. 52). 



Another anatomic fact seen also in the accompanying figure, concerns 

 the disposition of the muscular fibers of the auricle. These radiate in 

 bundles in a peculiar fan-shaped manner from a point which lies im- 

 mediately below the sinoauricular node to all parts of the superficies of 

 the right auricle. This point has been called the concentration point* 

 At the termination of the venae cavae, the muscle fibers are arranged more 

 or less circularly. 



Having become familiar with the disposition in the mammalian heart 

 of the primitive cardiac tissue, along which in the heart of the lower 

 animals we know that the heartbeat spreads, we may now proceed to 

 examine the evidence showing that this tissue is also responsible for the 

 origination and propagation of the beat in the heart of mammals. With 

 regard to the origin of the beat in a normally beating mammalian heart, 

 it is of course impossible to tell where this takes place. If the heart is 

 excised, however, it will continue to beat for a few moments, and as it 

 dies it will be observed that the power of contraction remains in the au- 

 ricular region, and particularly at the bases of the venae cavae, for a con- 

 siderable time after the ventricles have ceased to beat. This part the 

 ultimum moriens is situated in most hearts somewhat low r er than the 

 sinoauricular node. That it is the last part of the heart to cease con- 

 tracting does not necessarily mean that it is the part of the heart in 

 which the beat ordinarily originates ; it means simply that this is the 

 part of the auricle in which the power of contraction remains for the 



